It is generally known that maintaining laminar flow of air passing over an airfoil can improve the aerodynamics and performance of an aircraft. For example, it is known that delaying the transition of boundary layer airflow from laminar flow to turbulent flow over aerodynamic surfaces can reduce skin friction and reduce aerodynamic drag. One method of delaying the transition of airflow from laminar to turbulent flow is by installing a porous skin at critical areas of an airfoil such as along the leading edges of wings, tail surfaces and engine nacelles. The porous skin typically includes a large quantity of apertures or pores of relatively small size. The porous skin may also include narrow slots or elongated pores to provide porosity. The pores in the porous skin of a wing leading edge may be formed at diameters on the order of several thousandths of an inch (e.g., 0.0025″) or less and at spacings of tens of thousandths of an inch (e.g., 0.035″) between adjacent pores.
By applying a suctioning force to the porous skin, boundary layer airflow that is attached to the airfoil (i.e., along the attachment line) is drawn through the pores to stabilize the boundary layer against small disturbances which may grow and ultimately lead to early transition turbulence. The application of the suction force thins and robustifies the boundary layer velocity profiles. The net result is a delay in boundary-layer transition, a decrease in skin friction drag, and an increase in aerodynamic efficiency of the aircraft. The increase in aerodynamic efficiency may be especially noticeable at cruise altitudes for long distance flights wherein significant fuel savings may be achievable as a result of reduced aerodynamic drag.
One of the challenges preventing widespread implementation of laminar flow control systems of the suctioning type is contamination or blockage of pores which can occur under certain conditions. Such contamination may include atmospheric contamination and/or manmade contamination which can reduce the effectiveness of laminar flow control systems. For example, during takeoff and climb-out of an aircraft fitted with porous skins, precipitation in the form of rain or moisture in low-altitude clouds can fill the pores with water that will later freeze as the aircraft climbs into colder air. The frozen moisture blocks the pores and reduces the effectiveness of the suctioning system in maintaining laminar flow over the aircraft during cruise. Manmade contamination such as de-icing fluids applied during ground operations may also reduce the effectiveness of the laminar flow control system by clogging the pores with de-icing fluid.
The accumulation of frost on an aircraft may also reduce the effectiveness of a suctioning system by blocking the pores. Although frost accumulations on exterior skin surfaces of the porous skin may eventually sublimate away, moisture or liquid on the interior skin surfaces of the porous skin may become trapped in the pores and will remain as a result of the relatively small amount of surface area over which the sublimation occurs. Furthermore, local flow velocities inside the pores are relatively low and therefore insufficient to overcome surface tension resistance of the moisture trapped within the pores.
Prior art attempts at preventing clogging of pores include active purging systems wherein pressurized air is expelled or discharged outwardly through the pores. Purging systems may be activated prior to takeoff in anticipation of rain or moisture-laden clouds that may be encountered during climb-out. In this manner, such purging systems maintain the pores in an unblocked state and prevent the freezing of residual liquid that may be trapped within the pores. Although effective for their intended purposes, prior art purging systems suffer from several defects that detract from their overall utility.
For example, all known purging systems for use with laminar flow control systems of the suction type are active purging systems. Active purging systems require additional energy input in the form of engine bleed air or pumping machinery to provide the pressurized air for discharge through the pores of the laminar flow control system. The pressurized air may be drawn from engine compressors or other turbo-machinery. For example, pressurized air for the purging system may be provided by tapping a portion of the bypass flow of a high-bypass turbofan engine.
As may be appreciated, the system architecture of an active purging system such as one which draws pressurized air from an aircraft engine may be functionally and structurally complex. Such active purging systems require the installation of components and machinery which may increase complexity and add to fabrication and operational costs. Even further, the components of an active purging system may result in an increase in weight of the aircraft which may detract from gains in fuel efficiency otherwise attainable with the laminar flow control system.
Even further, some aircraft such as commercial airliners are increasingly fabricated without significant bleed air extraction from the engine. Although bleed air extraction has been conventionally used for various aircraft systems such as for cabin pressurization and in-flight de-icing, many modern aircraft are now substituting electrical power for conventional engine-generating pneumatic power (i.e., bleed air) in order to limit the amount of pneumatic power that is extracted from the engines such that the engines may produce maximum thrust. As such, conventional engine bleed air may be unavailable for powering an active purging system on future aircraft.
As can be seen, there exists a need in the art for a purging system for use with a laminar flow control system which avoids the complexity and weight associated with active purging systems. Furthermore, there exists a need in the art for a purging system for a laminar flow control system which requires a minimal amount of maintenance and which is low in cost.